Recently a great deal of research effort in the coatings industry has been focused on mar and abrasion resistance of transparent coatings. While these two terms are often used interchangeably, the physical and mechanistic events are quite different. Mar is a near-surface phenomenon involving small scratches, usually without significant removal of material. Abrasion involves much more severe damage and often entails significant loss of coating material. As such, mar resistance is influenced primarily by surface properties while abrasion resistance is controlled by bulk properties. The chemical attributes that influence these mechanical properties are often divergent. For example, acrylic urethane clearcoats can have excellent abrasion resistance but poor mar resistance. On the other hand, acrylic melamine coatings can have good mar resistance and average to poor abrasion resistance. One common example of mar damage is gloss loss on an automobile finish. The high gloss finish of a new car becomes dull and hazy with time due to fine scratches and abrasions. The majority of this damage is caused by the scrubbing action of cloths or bristles used in automatic car washes. Abrasion damage is more commonly seen in floor coatings, and in its most severe form the substrate may become exposed. Burnishing of highly pigmented coatings is less well defined in the literature but contains elements of mar and abrasion as well as interfacial adhesion. There are at least four different wearing mechanisms that can contribute to burnishing in coatings containing fillers or pigments. 1) Removal of film at the air/surface interface, 2) abstraction of the filler by cohesive binder failure, 3) abstraction of filler by adhesive failure and/or 4) filler wear. Which mechanism predominates may vary depending on the coating, abrasion conditions and outdoor exposure. Ideally a burnish resistant coating has good abrasion resistance, good mar resistance, and pigment particles with excellent compatibility with the binder.
Camouflage aircraft typically utilize low gloss pigmented coatings to achieve many of their performance requirements. These low gloss or matte finishes exhibit physically rough surfaces, which, ideally, diffusely reflect visible light. These physically rough surfaces are difficult to clean and often require scrubbing with an abrasive pad, which over time can increase the uniformity of coating reflectance, called burnishing. The increase in reflection is perceived visually as a color shift resulting in areas of higher contrast. Mechanically, this increase in gloss or burnishing is caused by abrasion of the rough paint surface producing a smoother surface with more uniform reflection.
It would be desirable to develop curable film-forming compositions which can be repeatedly cleaned with detergents and abrasives without increasing the reflectance of the coating, to be used in methods of improving the burnish resistance of metal substrates.